As telecom operators are struggling to accommodate the existing demands of mobile users, new data consuming applications are emerging in the daily routines of mobile users (e.g., proximity-aware services). Moreover, 4G cellular technologies (WiMAX and LTE-A), which have extremely efficient physical and MAC layer performance, are still lagging behind mobile users' booming data demand. One of the most promising approaches to boost mobile capacity is to reuse efficiently existing frequency bands. This can be accomplished by several options, such as densification of the base stations and deployment of small cells underlying the conventional cellular networks, and using cognitive radio and spectrum sharing. Device to-device (D2D) communication is one such paradigm that appears to be a promising component in the next generation cellular technologies. In fact, a Third Generation Partnership Project (3GPP) study group is investigating D2D communications in cellular networks.